This invention relates in general to improvements in dry separation processes for the physical separation of different species of the material constituents of a mixture of particles, more particularly to new methods and means for increasing the respective concentrations of separated species of such constituents. The invention is applicable to a wide variety of physical mixtures, such as separating ice crystals from pulverized, frozen, aqueous solutions, as well as to the benefication of ores. It has been found to be particularly useful in the separation of impurities from coal, i.e.: coal benefication.
The constituents of coal which are considered to be "impurities" include those containing sulfur and some minerals which form non-combustible ash. Ash-forming constituents coat, foul and drastically reduce the efficiency of heat transfer in boilers, in addition to polluting the environment. Sulfur-bearing constituents contribute to environmental pollution, one form of such pollution being commonly referred to as "acid rain". As found in its natural state, coal contains varying proportions of these impurities, the proportions in any one deposit depending on the geological history of that deposit.
Coal benefication begins with a process of crushing, pulverizing, or comminuting coal, to break pieces of coal down to particles of smaller and smaller sizes, which frees the constituents from one another and thereby enables them to be separated. Eventually, this process yields particle sizes so small that the cost and difficulty of handling the product becomes formidable barriers to further progress. The finer the coal is comminuted the greater is the portion of the impurity constituents that can be physically freed for eventual separation from the coal. Finely-comminuted coal particles can be confined in a liquid slurry for further treatment, but that approach requires the use of water or other liquid, which adds to the cost and complexity of the separation process and therefore is not economically or logistically desirable on a commercial scale. Dry-separation processes involve the steps of electrically charging the particles in a mixture and thereafter separating charged particles in an electric field in a gaseous medium. However, the dry-separation processes that are now available to commerce and industry do not deal efficiently with the finer-sized constituents of particulate mixtures (e.g.: smaller than 37 microns, or 400 mesh).
It is customary in the known processes to first impart electric charges to the different species of constituents, and then to separate the species in an electric field on the basis of different polarities, but the efficiency of this second step depends on the particles retaining their respective charges until they come under the influence of the electric field. The present invention introduces a new dry-separation process which overcomes these deficiencies in a new way.
Similar problems are encountered in the beneficiation of phosphate ores, which are mined in a matrix comprised of pieces of phosphate rock and silica admixed in a clay-like material known as "slimes." The matrix material must be disintegrated as much as possible in order to efficiently recover phosphate rock. In the process, significant quantities of ultra-fine particles (slimes) are produced.
In the preparation of concentrates of foods and other substances from liquid solution and slurries it would be useful to concentrate the substances carried in the liquid by freezing the liquid and filtering out particles in its frozen state; for example, to concentrate a fruit juice by freezing and filtering out ice crystals. Present technology removes water by evaporation, which consumes 1000BTU/1b, whereas freezing requires only 144 BTU/1b. The present invention is useful in a freezing process followed by pulverization of the frozen liquid and then removal of the particles of frozen liquid in a dry separation process using electrostatic separation forces.
This invention teaches new methods and means for electrically charging and separating different species of the constituents of coal and other ores, solutions and slurries, including powder-like ultra-fine particles sizes (e.g.: smaller than 100 microns), and for electrically charging a mixture which includes such ultra-fine particles, so as to enable particles of impurities and particles of coal, phosphate, solute or other desired component, or species of constituents of any such mixture, to be separated from each other in an electric field more efficiently than has heretofore been achieved on a commercial scale.